A known technique for active stereo camera sensing uses an infrared (IR) projected pattern in conjunction with one or more cameras capable of detecting IR. As higher resolution three-dimensional (3D) sensors are devised to operate at larger and larger field of views and distances, and with higher and higher resolutions, the total IR illumination power needs to increase.
IR lasers in conjunction with one or more diffractive optical elements (DOEs) have been used as power sources to produce such patterns. However, at such power levels, the intensity of the DOE's non-diffracted light (or zero-order light, usually a fixed fraction of total energy) is an eye-safety hazard.
One common way to reduce the intensity of the non-diffracted light is to have two spot-generating DOEs in a series. The first DOE creates the complex dot pattern, and the second one replicates it by a small number N×N, e.g., 3×3. As the non-diffracted light is a fixed ratio of the total power, (e.g., represented by α), the final power of the non-diffracted light is α/N2. Thus, the second DOE effectively decreases the intensity power by the factor N2. This resulting decrease in power is counter to the need for increased IR illumination power.